Facebook became the first to release aggregate numbers of requests, saying in a blog post that it received between 9,000 and 10,000 US requests for user data in the second half of 2012, covering 18,000 to 19,000 of its users' accounts.

Key points

US authorities made between 9,000 and 10,000 requests to Facebook for user data in the second half of 2012

Microsoft says it received requests for information on about 31,000 consumer accounts

Google is negotiating over whether it could only publish a combined figure for all requests

Until now, all information about requests were deemed secret

The majority of those requests are routine police inquiries, a person familiar with the company said, but under the terms of the deal with Justice Department, Facebook is precluded from saying how many were secret orders issued under the Foreign Intelligence Surveillance Act.

Until now, all information about requests under FISA, including their existence, were deemed secret.

Microsoft said it had received requests of all types for information on about 31,000 consumer accounts in the second half of 2012.

In a "transparency report" Microsoft published earlier this year without including national security matters, it said it had received criminal requests involving 24,565 accounts for all of 2012.

If half of those requests came in the second part of the year, the intelligence requests constitute the bulk of government inquiries. Microsoft did not dispute that conclusion.

Google said that it was negotiating with the government and that the sticking point was whether it could only publish a combined figure for all requests.

It said that would be "a step back for users" because it already breaks out criminal requests and National Security Letters, another type of intelligence inquiry.

Internet giants fighting expanded public backlash over Prism

What is PRISM?

Eavesdropping program used by US intelligence agencies

Allegedly allows the National Security Agency (NSA) and the FBI direct access to servers to track an individual's web presence

Washington Post says PRISM program is the most significant contributor to US president Barack Obama's daily briefings, accounting for one in seven intelligence reports

Facebook, Google and Microsoft had all publicly urged the US authorities to allow them to reveal the number and scope of the surveillance requests after documents leaked to the Washington Post and the Guardian suggested they had given the government "direct access" to their computers as part of a National Security Agency program called Prism.

The disclosures about Prism, and related revelations about broad-based collection of telephone records, have triggered widespread concern and congressional hearings about the scope and extent of the information-gathering.

The big internet companies in particular have been torn by the need to obey US laws that forbid virtually any discussion of foreign intelligence requests and the need to assuage customers.

"We hope this helps put into perspective the numbers involved and lays to rest some of the hyperbolic and false assertions in some recent press accounts about the frequency and scope of the data requests that we receive," Facebook wrote on its site.

Facebook said it would continue to press to divulge more information.

Uncertainty over what data can be looked at

It is believed that FISA requests typically seek much more information, but it remains unclear how broad the FISA orders might be.

Several companies have said they had never been asked to turn over everything from an entire country, for example. However, the intelligence agencies could ask for all correspondence by an account holder, or even all correspondence from the users' contacts.

Among the other remaining questions are the nature of court-approved "minimisation" procedures designed to limit use of information about US residents. The NSA is prohibited from specifically targeting them.

"If they are receiving large amounts of data that they are not actually authorised to look at, the question then becomes what are the procedures by which they determine what they can look at?" said Kevin Bankston, an attorney at the Centre for Democracy and Technology.

"Do they simply store that forever in case later they are authorised to look at it?"

In addition, some legal experts say that recent US laws allow for intelligence-gathering simply for the pursuit of foreign policy objectives, not just in hunting terrorists and spies.

Google, Facebook and Microsoft have already directly contradicted the Guardian and Washington Post reports about "direct access" to their servers.

Both newspapers have since backtracked, and it now appears that at least some of the companies allowed neither government-controlled equipment on their property nor direct searches without company employees vetting each inquiry.

Google has been the most forthright on the technology issue, saying that it provides information only on request via an old-school data-transfer protocol called FTP and that Google legal staff must approve each request.

Beyond that, it is now clear that many of the companies have objected, at times strenuously, to both individual requests and the broad sweep of the program. It remains unclear how successful they have been.

Wrestling over secret orders

The initial reports about Prism included an internal NSA slide listing the dates that each of nine companies began allowing Prism data collection, starting with Microsoft in 2007 and Yahoo in 2008.

The other companies include Apple, AOL and PalTalk as well as YouTube and Skype, which are owned by Google and Microsoft respectively.

Sources familiar with the conversations between the government and the internet companies say there are frequent disagreements over how to handle specific requests.

Only one company, Yahoo, is known to have taken the highly unusual step of appealing an order from the Foreign Intelligence Surveillance Court.

The company argued in 2008 that the order violated the Fourth Amendment protection against unreasonable searches and seizures.

Twitter, which has positioned itself as a hard-line defender of free speech and customer privacy, is still not participating in Prism. But people familiar with talks between the tech companies and the government said it will likely be forced to comply.